I’m very sorry to hear about the problems you’ve been having with your Mandarins, sir. It’s quite common for them to starve to death in the aquarium if they are kept in tanks that are too small, too new, or that otherwise lack a sufficient population of live copepods, amphipods, and other small crustaceans for them to feed on continuously. Here are some tips that explain the conditions the mandarins need in order to thrive in more detail.
I absolutely love the psychedelic coloration and peaceful nature of Mandarin dragonets! There’s no disputing that they are gorgeous little fishes and make ideal tankmates for seahorses in the right type of setup. They are docile, slow-moving, passive fish that are beautifully marked and very deliberate feeders. And they are quite hardy fish providing they can be fed properly. They have a heavy slime coat that seems to make them quite resistant to protozoan parasites such as Cryptocaryon irritans.
But in order to do well, mandarins need a large, well-established aquarium loaded with live rock that’s teeming with copepods and amphipods. As you know, mandarins must have continuous opportunities to graze on suitable live foods or they generally slowly waste away and starve to death. In the right system, they can thrive, and will often learn to take small pieces of frozen Mysis, but they do best in well-established reef systems or aquariums with at least 1 pound of live rock per gallon, a mature sand bed, and a refugium that can continually replenish the pod population in the tank. Those are typically the conditions that are necessary to assure they have adequate suitable live prey.
It’s going to be difficult to provide them with the conditions they need to thrive unless your aquarium is large enough (at least 50 gallons; 100 plus gallons would be preferable), chock-full of live rock, and has been up and running long enough to establish an abundant pod population. The live rock provides the shelter and feeding opportunities that the copepods and amphipods need in order to build up and maintain a breeding population. With several hungry seahorses competing with them for the available pods, it will be challenging for you to provide a Mandarin goby with enough to eat.
Under the circumstances, Dean, I would only consider keeping a Mandarin if you can equip the aquarium with a good refugium as well. As an example of the type of refuge that produces good results for mandarins and seahorses, Charles Delbeek likes to use colonies of glass shrimp and cleaner shrimp that in the refugium for his seahorse tank, where the regular reproduction of these hermaphroditic crustaceans will provide a continuous supply of nutritious nauplii for his ponies:
"There is a method that can be used to offer an occasional supply of live food for your sea horses. By setting up a separate system housing several species of shrimp such as the common cleaner shrimp (Lysmata amboinensis) or peppermint shrimp (lyse model wurdemanni), or Rhynchocinetes uritai or R. durbanensis, you can get a fairly regular supply of live shrimp larvae. These species are best to use since they can live in large groups and spawn on a regular basis. Such a system is commonly called a refugium. A refugium is a small (10-20 gallon) aquarium that contains live sand, live rock and/or macroalgae such as Caulerpa. It is plumbed such that water from your main system is pumped to the refugium and then returns via an overflow to the main tank. For this type of arrangement to work, the refugium must be slightly higher than the main tank. Shrimp are added to the refugium and within a few months they should start spawning and hatching eggs every few weeks. The larvae are then carried back to the main tank by the overflow, where they become a food source for your sea horses. Of course other life will also thrive in the refugium and it is not unusual for copepods, mysis and crab larvae to also be produced on a regular basis. The key to the refugium is to keep predators out of the system so that the smaller micro-crustacean population can thrive. You would need a fairly large and productive refugium to produce enough food to maintain even a pair of sea horses, so at best, a typical refugium can provide a nice source of supplemental live food; the basic daily diet still needs to be provided by you in the form of the frozen foods mentioned above." (Delbeek, November 2001, "Horse Forum," FAMA magazine)
Aside from the one Delbeek favors, refugia are available in a number of different designs. For example, there are easy-to-install external hang-on refugia and in-tank refugia as well as sump-style refugia that are mounted beneath the main. In the case of the latter, the refugium is installed exactly like any other sump. Here are a couple of online sites where you can look up more information on refugia, including articles explaining how to set up and install a refugium of your own:
Click here: Refugium Setups Information – From About Saltwater Aquariums
Click here: Refugiums
It sounds like you have basically the right set up for the Mandarin, since he is in a modified reef tank with live rock and live corals, but I would recommend establishing a refugium for your seahorse tank as described above, Dean. And in the meantime, I would try target feeding the Mandarin with some of the smallest bite-size frozen Mysis. The Mini Mysis from H2O Life or the miniscule Hikari frozen Mysis would be worth a try for this, sir.
Best of luck fattening up your Mandarin Dragonet, Dean!